Catalytic purification of terephthalic acid

ABSTRACT

Crude terephthalic acid is purified at elevated temperatures by contact in the presence of carbon monoxide with solid particles of an adsorptive agent having substantial carbon monoxide sorption capacity (e.g., palladium on a carbon support) to obtain a high recovery of the purified acid while effecting a high percentage conversion of the small amounts of paracarboxybenzaldehyde contaminating the crude acid. In at least some instances, small amounts of hydrogen may be charged to increase the catalyst life; and steam or another inert gaseous carrier is desirably employed in vapor phase operations.

United States Patent Arnold F. Stancell Highland;

Clairborne A. Duval, Jr, Westfield, both of NJ.

June 12, 1968 Sept. 21, 1971 Mobil Oil Corporation [72] Inventors 211Appl. No. [22] Filed [45] Patented [73] Assignee [54] CATALYTICPURIFICATION OF TEREPI-ITHALIC FOREIGN PATENTS 597,876 5/l960 Canada260/525 726,213 l/l966 Canada 260/525 OTHER REFERENCES Sabatier, LaCatalyse en Chimie Organique," 1920, pages 199- 200.

Primary ExaminerLewis Gotts Assistant Examiner-R. S. WeissbergAttorneys0lswald G, Hayes, Donald .L. Dickerson and James F. SnowdenABSTRACT: Crude terephthalic acid is purified at elevated temperaturesby contact in the presence of carbon monoxide with solid particles of anadsorptive agent having substantial carbon monoxide sorption capacity(e.g., palladium on a carbon support) to obtain a high recovery of thepurified acid while effecting a high percentage conversion of the smallamounts of para-carboxybenzaldehyde contaminating the crude acid. In atleast some instances, small amounts of hydrogen may be charged toincrease the catalyst life; and steam or another inert gaseous carrieris desirably employed in vapor phase operations.

CATALYTIC PURIFICATION OF TEREPHTI-IALIC ACID BACKGROUND OF THEINVENTION 1. Field of the Invention This invention relates to animproved method for the purification of crude terephthalic acidcontaminated with a minor proportion of p-carboxybenzaldehyde whereinthe latter is converted into substances which are readily separable fromthe terephthalic acid.

2. Description of the Prior Art Terephthalic acid is a compound ofincreasing commercial importance as exemplified by its use in largequantities in the production of fiber and film-forming polyesters, suchas polyethylene terephthalate of high molecular weight. For

many purposes, including the use of such polyesters in textile fibersand magnetic tape bases, an extremely high degree of polymer purity isnecessary to obtain the desired color, dyeing characteristics and/orexcellent physical and electrical properties. l-leretofore, mostcommercial production of such pure polyesters has involved an indirectroute of conversion of the terephthalic acid to its dimethyl ester andpurification of the ester by recrystallization or distillation prior totransesterification of the dimethyl ester with a glycol (e.g. ethyleneglycol) and polymerization of the transesterification product. Morerecently, it has been found advantageous to directly esterifyterephthalic acid of high purity with the glycol; hence efficient andeconomical methods for the purification of terephthalic acid per se arenow of prime importance.

Terephthalic acid can be manufactured by several processes known in theart, as exemplified by the catalytic oxidation of p-xylene according tothe processes described in U.S. Pat. Nos. 2,833,816, 2,853,514,3,036,122 and others.

The major impurities present in terephthalic acid prepared by thecatalytic oxidation of p-xylene result from incomplete oxidation; andthe crude product generally contains about 0.5 to 3 percent ofp-carboxybenzaldehyde as well as other intermediate oxidationbyproducts, usually including p-toluic acid. Leaching of this crudeproduct with hot acetic acid or another suitable agent may be employedto reduce the content of such impurities and provide a leached crudematerial typically containing about 0.2 to 1.5 percentp-carboxybenzaldehyde. Also, a much smaller amount of ash is typicallypresent in the form of one or more metal compounds derived from theresidue of a metal salt oxidation catalyst (e. g. cobalt acetatetetrahydrate) and/or the corrosion of process equipment, silica andorganic decomposition products.

A particularly troublesome byproduct of p-xylene oxidation isp-carboxybenzaldehyde which acts as a chain stopper during subsequentpolyesterification of the terephthalic acid and, either alone or incombination with other intermediate oxidation products, impartsundesirable coloration to the resulting polyester product. Moreover, itis difficult to remove this aldehyde compound from terephthalic acid byconventional techniques, particularly in meeting commercialspecifications for a maximum p-carboxybenzaldehyde content in the rangeof 15 to 50 parts per million (ppm) by weight.

Various procedures have been proposed for the purification ofterephthalic acid including fractional recrystallization, leaching,sublimation with fractional condensation and contact with ahydrogenation catalyst (e.g. palladium) in the presence of hydrogen, butmost of these methods have their limitations in respect toeffectiveness, cost or operating difficulties encountered.

SUMMARY OF THE INVENTION The present invention is a method for thepurification of crude terephthalic acids contaminated with a minoramount of p-carboxybenzaldehyde wherein a dispersion of the crude acidis treated in the presence of carbon monoxide by contact with theparticles of a solid adsorptive agent which is capable of absorbing asubstantial amount of carbon monoxide and the resulting treated producthas a substantially lower p-carboxybenzaldehyde content than the crudeacid.

Other aspects of the invention relate to suitable contact agents,especially the metals in Group VIII of the Periodic Table of Elements ofwhich palladium is preferred; treating the crude acid while in the vaporstate at a temperature above its dew point in the reaction mixture andbelow the temperature level at which substantial decomposition of theterephthalic acid occurs during the treatment; introducing a substantialamount of an inert gaseous carrier, such as steam, to entrain the fineparticles of the acid during vaporization; the subsequent condensationof the terephthalic acid to solid form in the substantial absence of thecontact material; the inclusion of gaseous hydrogen in the charge, andthe preferred proportions of various components of the charge.

These and other features of the invention as well as its objects andadvantages will be apparent to those skilled in the art uponconsideration of the general and detailed disclosure hereinafter.

The instant method involves highly selective catalysis in which a highdegree of conversion of the unwanted p-carboxybenzaldehyde occurs eventhough this substance is present in low concentration (e.g. usually lessthan about 2 percent of the crude material) and wherein there is aminimal conversion or degradation of the terephthalic acid undergoingpurification; accordingly a good yield of a product of high purity isobtainable. This is accomplished by dispersing the crude solidterephthalic acid either by vaporization or by solution in a suitablesolvent liquid and contacting the resulting dispersion at elevatedtemperature with both carbon monoxide and a Group VIII metal or othersolid adsorptive agent which is capable of adsorbing a substantialamount of carbon monoxide under the reaction conditions; and thiscontact treatment is followed by recovery of the purified terephthalicacid. In many instances, hydrogen may desirably be introduced into thereaction mixture, particularly for prolonging the effective life of thecontact. In the case of vapor state treatment, the use of an inertgaseous carrier material is often advantageous in facilitating thevaporization and transport of terephthalic acid, and steam is especiallysuitable for the purpose as it eliminates or minimizes any tendencytoward dehydration at high temperatures of terephthalic acid toanhydride material which usually has an undesirable color.

In a preferred embodiment in which sublimation is utilized, the crudeterephthalic acid is vaporized in a hot gaseous mixture containing asubstantial proportion of superheated steam, a smaller amount of carbonmonoxide and a still lower concentration of hydrogen; the resultingvapor mixture is contacted with palladium or another metal of Group VIIIas the active catalyst or contact agent which is advantageouslysupported on an inert solid carrier material; then after separation fromthe catalytic material, the effluent vapor is cooled to ef fectfractional condensation of purified terephthalic acid in solid formwhile the uncondensed gaseous material containing most of the conversionproducts formed in the treatment is withdrawn.

Although terephthalic acid sublirnes without melting, it becomes quitetacky and particles tend to agglomerate at temperatures of 450 F. andhigher; therefore, it is preferable in vapor phase operations to chargethe crude material in the form of finely divided particles in a nontackycondition, that is at ambient temperature or at an elevated temperaturewhich is below the tackiness range, into a stream of entrainer orcarrier gas which is flowing through a pipe or other conduit at atransport velocity whereby the solid acid particles immediatelyentrained and transported in suspension in the carrier gas.

In accordance with this invention, impure terephthalic acid is subjectedto contact with an absorptive solid material that is capable ofadsorbing a substantial amount of carbon monoxide and of selectivelypromoting the conversion of the minor amount of p-carboxybenzaldehydepresent rather than conversion of the large amount of terephthalic acidpresent. The active component of the contact agent may comprise one ormore of the metals of Group VIII of the Periodic Table of Elements inmetallic or elemental form. Accordingly, the catalyst may be nickel orother metals of higher atomic number and higher molecular weight in thatgroup, as exemplified by ruthenium, osmium, iridium, rhodium andplatinum, along or in mixtures or in alloys with these or other metals.Palladium is preferred in the practice of this invention.

While a finely divided metal alone may be used as the contact material,its effectiveness is usually enhanced when it is disposed on solidparticles of an inert carrier or support material as this generallyproduces a substantial increase in the surface area of active metalexposed to the reaction mixture. For example, a fine palladium powdertypically has a surface area of about 30 square meters per gram, whereasin the extended form of a palladium-on-carbon catalyst, the surface areais about 120 square meters per gram of palladium. Carbonaceous materialssuch as activated carbon, powdered charcoal, etc. are particularlysuitable supporting materials for the active component but various otherinert carrier materials,

as exemplified by alumina or silica-alumina, may also be used. Thecontent of the active agent in the contact material may range from about0.001 to percent or more of the total weight, and very good results areobtainable with contact material containing from about 0.1 to about 10percent by weight of palladium on powdered activated carbon. Thepreparation of such composite contact agents is well known and a sizablenumber are readily available in prepared form as they are frequentlyused in the catalytic processing of petroleum hydrocarbons,hydrogenation, etc. In some cases, it may be desirable to further lowerthe concentration of the active metal by mixing the composite catalystwith a large proportion of another inert substance such as sand, marblechips, glass beads, etc.

In this process, the contact agent may be employed as a fixed and stillbed of solid particles through which the mixture containing carbonmonoxide and the dispersion of crude terephthalic is flowing, or finelydivided particles of the contact material of suitably small size may besuspended in and carried by either a gaseous or liquid stream containingthe terephthalic acid. For illustration, solid particles of contactagent suspended in the vapor stream desirably have an average particlesize smaller than 600 microns. Also, in vapor phase operations, thegaseous charge mixture may be passed through a fixed, dense phasefluidized bed of the contact agent; alternatively, it is possible toprovide countercurrent contact of larger particles of the contact agentfalling downwardly through a vapor charge flowing upward at a moderatevelocity by selecting a contact material of suitable particle size andweight as well as using a sufficiently low gas velocity in a reactionchamber provided with means for charging particulate solids at the upperend and their removal at the lower end.

In fixed bed processing, two or more reaction chambers arranged inparallel may often be desirable so that continuous operations may bemaintained by diverting the charge mixture to a second reactor while thebed of contact material in the first reactor is either being regeneratedor replaced with a bed of fresh contact material.

When the treatment is carried out with the contact agent entrained orsuspended in the dispersion of crude terephthalic acid, the solidcontact agent can be introduced by any convenient method, such as bymixing it with the crude acid prior to vaporization, or by mixing itwith the solution after the crude acid has been dissolved in a solventfor a liquid phase treatment, or by injecting it, together with eitherthe carbon monoxide or entrainer gas or both.

Although carbon monoxide is essential in the process of this invention,it may often be employed in relatively small amounts. A substantialdegree of p-carboxybenzaldehyde conversion or elimination is obtainablewhen only about 0.3 standard cubic feet (s.c.f.) of carbon monoxide isintroduced per pound of crude acid charge. However, it is usuallypreferable to charge between about 1 and 10 s.c.f. per pound of thecrude acid when a product of maximum purity is sought. Still largerproportions of carbon monoxide may be charged, and this is oftenadvisable when gaseous hydrogen is included in the reaction mixture asdescribed hereinafter. In instances where carbon monoxide is charged ata high rate, consideration should be given to recovering it from thereaction products and recycling it to the reactor unless a substantialsupply of this gas is readily available at low cost. If necessary, therecycled carbon monoxide may be purified by such techniques as coolingand/or washing to condense and/or wash out any impurities which mighttend to contaminate the terephthalic acid.

The carbon monoxide may be introduced as part of the gaseous carriermaterial for entraining crude acid particles or it may be chargeddirectly into the reactor. Carbon monoxide already adsorbed on thecontact agent may constitute the entire supply of carbon monoxide forrelatively brief operating periods. For instance, the contact agent maybe pretreated with carbon monoxide alone or in mixtures of nitrogen orsteam containing as little as 5 mol percent carbon monoxide attemperatures of 250 F. and higher for periods ranging upward from 5minutes to several hours prior to introducing the crude acid to betreated.

In fixed bed operations, a gradual decrease in p-carboxybenzaldehydeconversion has been observed as the operation progresses and this hasbeen attributed to a decline in catalyst activity. While the specificcause of this effect has not been determined, it has been found that thecatalyst activity and consequently the life of the contact material canbe extended or prolonged greatly by incorporating hydrogen gas in thereaction mixture in an amount which is usually considerably less thanthe carbon monoxide but generally greater than about 0.2 s.c.f. perpound of crude acid. The decline in activity of the contact agent isconsiderably more pronounced when the terephthalic starting material isof relatively low purity, as exemplified by having a content of morethan 1 percent p-carboxybenzaldehyde and an ash containing more thanabout 50 ppm. of cobalt calculated as the metal. In purifying theserelatively impure crudes, it is usually desirable to include betweenabout 1 and 10 s.c.f. of hydrogen per pound of crude acid in the chargemixture, and the volumetric or molar ratio of carbon monoxide tohydrogen in the reaction mixture is desirably above 10:1, and preferablyabove 20: l. The hydrogen may be introduced in the same manner as thecarbon monoxide and when carbon monoxide is recycled back to theprocess, any hydrogen present will be a component of the recycle gas.

In vapor phase operations, it is generally desirable for a number ofreasons to use a gaseous entrainer or carrier which is inert ornonreactive with the crude acid, the carbon monoxide and the catalyticmaterial. The gaseous material is customarily preheated and thusprovides at least part of the heat of sublimation of the crude acid. Inaddition, it is desirably introduced at transport velocity, that is agas velocity sufficient to immediately entrain and transport the crudeacid particles in a suspension of either the dilute or dense phase type.However, it is desirable to have the crude acid particles in dilutephase suspension at the time when most of the acid is vaporized, usuallyin a heated pipe coil, and this may be accomplished by introducing moreof the gaseous entrainer downstream of the point where the crude acidparticles were charged. During vaporization, the gaseous entrainer hasanother desirable effect in that its partial pressure lowers thetemperature required for vaporizing terephthalic acid. Nitrogen or otherinert gases may be utilized as the entraining agent, but steam,desirably in superheated form, is greatly preferred because the moisturetherein minimizes the tendency of terephthalic acid to dehydrate at hightemperatures and also from a standpoint of economy. The steam, ofcourse, may be introduced in admixture with other inert gases. Whensteam is employed for the purpose, it is desirable to have at leastabout 0.03 mol of steam present per pound of terephthalic acid (5:1steamzacid molar ratio). At the time most of the acid is vaporized, atleast double this amount of steam is present in preferred embodiments ofthe invention. While the charging rate may be one or more mols of steamper pound of the acid, such high steam rates are generally uneconomical.

For vapor phase treatments, the reaction temperature should bemaintained above the dew point of the gaseous reaction mixture and belowthe level at which substantial decomposition of terephthalic acid beginsto occur. In general, temperatures of about 600 to 800 F. are suitablefor the purpose. In the case of liquid phase treatments, temperatures inthe range of about 75 to 550 F. may be utilized.

Vapor phase reactions according to the present process may be carriedout over a wide range of elevated pressures extending up to 100 or morepounds per square inch gage pressure, but atmospheric or slightsuperatmospheric pressure are generally preferable to minimize theoperating difficulties in these high temperature reactions. In the caseof liquid phase treatments wherein the crude terephthalic acid isdissolved in water or a suitable organic solvent, such as acetic acid, asubstantial superatmospheric pressure is maintained on the reactionvessel in order to keep the solvent in the liquid phase at the selectedreaction temperatures.

The residence time for contact of the dispersion of the crudeterephthalic acid with a solid contact agent and carbon monoxide isdependent upon a number of factors including the reaction temperatureand the particular catalytic agent. In general, the residence time basedon volumetric flow may be between about 0.01 and seconds. In vapor phasetreatments with a supported palladium catalyst either suspended in thegaseous charge mixture of blended with inert solid particu' latematerial in a fixed bed, a residence time between about 0.1 and 5seconds is preferable. As a general rule, it is desirable to correlatethe contact or residence time with the reaction temperature in order toprovide as low a reaction temperature as possible in order toeliminateany possibility of discoloring the terephthalic acid product.

Following contact treatment of the mixture containing the crudeterephthalic acid, the treated material is preferably filtered before itis condensed. In process embodiments in which the contact agent isdispersed and carried by the mixture, such filtration is especiallydesirable in order not only to separate the contact material and ashfrom the purified acid product but also to accumulate a substantiallayer or bed of particles of contact material on the filter surface.There are indications that substantial purification of the reactionmixture occurs during its passage through such a layer in the case ofvapor phase treatments with entrained contact material. Any suitablefiltering means can be employed for separation of the contact agent orothersolid material from the treated vapor, with specific typesincluding porous metal, woven metal screens, ceramic mesh and glasscloth filters.

Following contact with the contact solids and preferably afterseparation therefrom, e.g. by the aforedescribed filtration step, thevaporized terephthalic acid in the resulting vaporous product can beseparated therefrom by condensation. Fractional rather than totalcondensation is preferred in order that the purified terephthalic acidmay be recovered while most of the more volatile components of thetreated vapor, particularly conversion products formed in the catalytictreatment, are withdrawn in the uncondensed vapor. Although condensationof the acid can be carried out by any suitable technique, includingcooling by heat exchange, it is often desirable to bring about thedesired condensation by combining a cooling medium with the mixturecontaining the vaporized terephthalic acid. Such a cooling medium shouldbe inert to terephthalic acid at the temperatures encountered in thepresent process, and is advantageously similar in nature to the inertgaseous medium used in preferred embodiments of this invention. Thus, itis generally satisfactory to inject a suffi cient quantity of a coolingmedium in the form of a water spray and/or relatively low temperaturesteam into the vaporous product mixture containing the terephthalic acidvapor to condense a substantial proportion of the acid vapor thereinwithout causing undesirable condensation of other constituents of theprocess stream, e.g., steam and impurities which remain in the vaporphase at temperatures lower than the condensation point of terephthalicacid. For example, a temperature of between about 350 and about 570 F.,and more specifically between 400 and 550 F. is generally preferred whencondensation is carried out at approximately atmospheric pressure,although the condensation can be carried out by cooling the vaporousproduct mixture to any temperature which is low enough to condenseterephthalic acid from the mixture. Following condensation, the solidterephthalic acid product can be separated from the cooled mixture byany appropriate method, e.g., by the use of a cyclone separator, filteror bag collector.

The terephthalic acid thus separated after the aforementioned treatmentwith the contact solids has been found to contain substantially smalleramounts of the impurities than the quantities originally present in thecrude terephthalic acid. For example, concentrations ofpara-carboxybenzaldehyde in crude terephthalic acid have in many casesbeen reduced by more than percent by the instant process.

In treatments according to the present invention, small quantities ofcarbon dioxide and benzene have been found in the reaction products and,in at least some instances, the products contained a greater quantity ofbenzoic acid than was present in the charge. Also, there are indicationsthat carbon monoxide is adsorbed on the contact agent at elevatedreaction temperatures. However, this process should not be regarded aslimited to any particular theory inasmuch as the reaction mechanism isnot yet fully understood.

It is further contemplated that crude acid may be subjected to two ormore of the contact treatments described herein in instances where thecrude terephthalic acid contains an unusually high concentration ofp-carboxybenzaldehyde or where a product of extremely high purity issought.

DESCRIPTION OF SPECIFIC EMBODIMENTS For a better understanding of thenature, objects and advantages of this invention, reference should behad to the following illustrative examples in which all proportions areset forth in terms of weight and all temperatures as degrees Fahrenheitunless otherwise indicated herein.

EXAMPLE 1 Crude terephthalic acid with a p-carboxybenzaldehyde contentof 13,532 p.p.m. produced by the cobalt-catalyzed oxidation of pxyleneand composed of solid particles averaging than about 5 pounds per squareinch gage pressure) amounts of 0.09 mol per pound of the crude acid(15:1 molar ratio). While being conveyed in suspension in the hotentrainer gas mixture, the solid panicles of the crude acid aresubjected to further heating while passing through a transfer linewithin a gas-fired furnace in which the catalytic reactor is located,and vaporization of the crude acid is complete prior to reaching thereactor.

Supported on a ZOO-mesh metal screen in the reactor is a fixed bed of0.9 inch diameter and 6 inch depth containing 0.5 gram of apalladium-on-carbon catalyst in uniform physical admixture with 85 gramsof glass beads of 470 micron average diameter to effectively dispersethe catalyst. The active adsorptive agent or catalyst of the contactagent is the 5 percent palladium metal which is deposited on powderedactivated.

carbon particles having an average size of the order 'of 40 microns insuch manner as to provide an extended surface area of about squaremeters per gram of palladium. Prior to charging the terephthalic acidvapor to the reactor, the contact mass is preheated to operatingtemperature while the encontact mass which is maintained at anaveragetemperature of 720 F., and the residence time, based on volumetric flow,of vapor in the bed is 1.2 seconds.

The effluent gaseous mixture from the reactor is subjected toessentially total condensation of its.content of normally.

solid and normally liquid components by cooling first to a temperatureof about 120 F. at atmospheric pressure in a waterjacketed condenser andthen to about 80 F. in an air-cooled condenser from which theuncondensed vapors or gases are vented after passing through a filterpaper which prevents the loss of any entrained solid particles.

After combining the material which collected in both condensers duringthe entire run, it is found thatthe total weight of solid materialrecovered amounts to 93.5 percent of the charge This product containsonly 14 p.p.m. of p-carboxybenzaldehyde which is indicative of a 99.9percent conversion of the latter substance during the treatment. A minoramount of product, 1.4 percent by weight, is water soluble and dissolvesin a large excess of water at room temperature. The terephthalic acidrecovery is 94.4 percent of the original terephthalic acid content ofthe charge.

EXAMPLE 2 Example 1 is repeated using the same feedstock under the samereaction conditions in all respects except for the pretreatment of afresh batch of the same palladium-carbonglass contact mass. Immediatelyprior to charging the acid to be purified, a mixture of equal volumes ofcarbon monoxide and superheated steam is passed through he catalyst bedfor a period of one hour while the catalyst bed is maintained at atemperature of 500 F.; then the reactor is rapidly heated to bring thebed temperature up to 720 F. and the flow of the vapor mixturecontaining terephthalic acid into the reactor is started.

In this run, the solid product recovered amounts to 97.0 percent of theweight of the charge and it contains only 95 p.p.m. ofp-carboxybenzaldehyde which is equivalent to 99.3 percent conversion ofthe latter substance; the water soluble material amounts of 1.59percent. More significantly, the recovery of terephthalic acid isincreased to 98.2 percent of the content of terephthalic acid in thecrude acid charge. Thus, a more selective purification is obtained withconsiderably less of the undesirable degradation of terephthalic acidtaking place in example 2 while the degree of conversion of theundesired p-carboxybenzaldehyde is similar to that of example 1.

In comparing examples 1 and 2, it is evident that a charge of only 0.6s.c.f. of carbon monoxide per pound of terephthalic acid is insufficientfor optimum purification. More importantly, the improved selectivity ofexample 2 is attributed to the presence in the reaction of additionalcarbon monoxide previously adsorbed on the palladium during thepretreatment and available from that source for a considerable periodduring the reaction. In view of the absence of vaporized crude acidduring pretreatment, the pretreatment is of particular interest -inindicating that a substantial amount of carbon monoxide is adsorbed onthe solid Contact agent.

EXAMPLE 3 Example 2 is repeated using another batch of the same catalystpretreated in the same manner and employing the same reaction conditionsexcept for increasing the carbon monoxide content of the gaseousentrainer mixture to 1.0 s.c.f. per pound of crude acid charged. Theresidence time is 1.3 seconds.

The results are generally similar to those obtained in Example 2 withthe total recovery of solid material amounting to 97.3 percent andcontaining 135 p.p.m. of p-carboxybenzaldehyde and representing a 99percent conversion of this impurity. The water soluble product amountedto 1.58 percent.

EXAMPLE 4 The procedure of Example 1 is repeated in all respects using afresh batch of the same contact mass without pretreatment, except forincreasing the carbon monoxide content of the entrainer gas to 8 s.c. f.per pound of the crude terephthalic acid. The residence time here is 1.0second.

Excellent results are obtained with a total recovery of solids amountingto 99 percent, and there is 1.17 percent by weight of water solubleproduct The solid product has a content of only 27 p.p.m. ofp-carboxybenzaldehyde which is indicative of a conversion of 99.8percent of this material. The terephthalic acid recoveryamounts to 100percent.

EXAMPLE 5 Using the same apparatus and 0.6 gram of the same 5 percentpalladium on carbon catalyst mixed with 85 grams of the same glass beadswithout pretreatment, a crude terephthalic acid of higher purity istreated according to the present invention. This product of p-xyleneoxidation has a lower p-carboxybenzaldehyde content of 1140 p.p.m. aswell as about 0.8 p.p.m. of cobalt and 1 p.p.m. of iron (calculated asmetals) in an ash content of about p.p.m.

The crude acid is fed at a rate of 35 grams per hour into a streamcontaining 0.22 mol of superheated steam and 2.8 s.c.f. of carbonmonoxide per pound of the crude acid. The residence time based onvolumetric flow is 0.5 seconds as the gaseous reaction material flowsdown through the bed of contact material maintained at an averagetemperature of 705 F.

A highly selective purification occurs and a good yield is obtained asevidenced by a total solids recovery of 96.8 percent of material havinga p-carboxybenzaldehyde content of only 52 p.p.m. (95 percentconversion) as determined after combining the entire solid condensatesfrom both condensers at the end of a run of substantial duration.Terephthalic acid recovery amounts to 96.8 percent of the content ofthis material in the charge.

EXAMPLE 6 s.c.f. of hydrogen, 5.3 s.c.f. of carbon monoxide and 0.25 molof superheated steam per pound of crude acid charged.

The total solids recovery in both condensers is found to be 93 percentof the charge, the recovery of terephthalic acid is 93 percent andproduct contains only 34 p.p.m. of p-carboxybenzaldehyde (97 percentremoval).

EXAMPLE 7 A low grade crude terephthalic acid with high contents of bothp-carboxybenzaldehyde (1.4 percent) and ash containing 73 p.p.m. ofcobalt (calculated as the metal) is purified in the same apparatus usinga fresh batch of the same contact mass employed in example 5 withoutpretreatment.

The crude acid is charged at a rate of 21.8 grams per hour into agaseous entrainer stream flowing at a rate which provides 2.1 s.c.f. ofhydrogen, 62.6 s.c.f. of carbon monoxide and 0.52 mol of superheatedsteam per pound of crude acid.

The adsorption bed is maintained at an average temperature limited inany particulars except as may be recited in the appended claims orrequired by the prior art.

We claim:

1. A process for the purification of terephthalic acid which comprisestreating a crude terephthalic acid vapor containing a minor proportionof p-carboxybenzaldehyde in the presence of carbon monoxide by contactwith a solid particulate adsorptive agent having a substantial carbonmonoxide sorption capacity to produce purified terephthalic acid ofsubstantially lower p-carboxybenzaldehyde content than said crude acid.

2. A process according to claim 1 in which gaseous hydrogen is presentduring said treatment and said adsorptive agent is situated in a fixedbed.

3. A process according to claim 1 in which said crude terephthalic acidis in the vapor state at an elevated temperature insufficient for thesubstantial decomposition of terephthalic acid during said treatment,and the treated gaseous product mixture is thereafter cooled to condenseat least a substantial proportion of the terephthalic acid vapor as apurified solid.

4. A process according to claim 3 in which the charge contains at leastabout 0.3 s.c.f. of carbon monoxide, at least about 0.2 s.c.f. ofhydrogen and at least about 0.03 mol of steam per pound of said crudeacid in a mixture wherein the carbon monoxide: hydrogen molar ratio isat least about 20:1.

5. A process for the purification of terephthalic acid which comprisestreating a crude terephthalic acid vapor containing a minor proportionof p-carboxybenzaldehyde by contact with a metal of Group VIII of thePeriodic Table of Elements in the presence of carbon monoxide to producepurified terephthalic acid of substantially lower p-carboxybenzaldehydecontent than said crude acid.

6. A process according to claim 5 in which gaseous hydrogen is presentduring said treatment.

7. A process according to claim 5 in which said metal is contained in afixed bed.

8. A process according to claim 5 in which said metal comprisespalladium supported on particles of an inert solid carrier material.

9. A process according to claim 5 in which said metal comprisespalladium supported on finely divided carbon particles.

10. A process according to claim 5 in which the charge contains at leastabout 0.3 standard cubic foot of carbon monoxide per pound of said crudeacid.

11. A process according to claim 5 in which the charge contains at leastabout 0.3 standard cubic foot of carbon monoxide and at least about 0.2standard cubic foot of gaseous hydrogen per pound of said crude acid.

12. A process according to claim 11 in which the molar ratio of carbonmonoxide and hydrogen in the charge is at least 10:1.

13. A process according to claim 5 in which said crude terephthalic acidis in the vapor state at an elevated temperature insufiicient for thesubstantial decomposition of terephthalic acid during said treatment,and the treated gaseous product mixture is thereafter cooled to condenseat least a substantial proportion of the terephthalic acid vapor as apurified solid.

14. A process according to claim 13 in which said metal is suspended infinely divided particulate form in the gaseous mixture during saidtreatment and thereafter separated from said mixture prior to condensingthe terephthalic acid.

15. A process according to claim 13 in which said crude terephthalicacid vapor is mixed with a substantial amount of an inert gaseoussubstance.

16. A process according to claim 15 in which vaporized crudeterephthalic acid is treated by contact at a temperature between about600and 800 F. and a residence time between about 0.1 and 5 seconds withan agent comprising between 0 about 0.001 and 15 percent by weight ofpalladium supported in extended surface area form on finely dividedcarbon particles in the presence of at least about I s.c.f. of carbonmonoxide at least about 0.5 s.c.f. of hydrogen and at least about 0.06mo of steam per pound of said crude acid 1n a mixture wherein the carbonmonoxide: hydrogen molar ratio is at least about 20:1, and the resultingtreated gaseous mixture is thereafter cooled in the absence of saidagent to condense at least a substantial proportion of terephthalic acidvapor as a purified solid.

2. A process according to claim 1 in which gaseous hydrogen is presentduring said treatment and said adsorptive agent is situated in a fixedbed.
 3. A process according to claim 1 in which said crude terephthalicacid is in the vapor state at an elevated temperature insufficient forthe substantial decomposition of terephthalic acid during saidtreatment, and the treated gaseous product mixture is thereafter cooledto condense at least a substantial proportion of the terephthalic acidvapor as a purified solid.
 4. A process according to claim 3 in whichthe charge contains at least about 0.3 s.c.f. of carbon monoxide, atleast about 0.2 s.c.f. of hydrogen and at least about 0.03 mol of steamper pound of said crude acid in a mixture wherein the carbon monoxide:hydrogen molar ratio is at least about 20:1.
 5. A process for thepurification of terephthalic acid which comprises treating a crudeterephthalic acid vapor containing a minor proportion ofp-carboxybenzaldehyde by contact with a metal of Group VIII of thePeriodic Table of Elements in the presence of carbon monoxide to producepurified terephthalic acid of substantially lower p-carboxybenzaldehydecontent than said crude acid.
 6. A process according to claim 5 in whichgaseous hydrogen is present during said treatment.
 7. A processaccording to claim 5 in which said metal is contained in a fixed bed. 8.A process according to claim 5 in which said metal comprises palladiumsupported on particles of an inert solid carrier material.
 9. A processaccording to claim 5 in which said metal comprises palladium supportedon finely divided carbon particles.
 10. A process according to claim 5in which the charge contains at least about 0.3 standard cubic foot ofcarbon monoxide per pound of said crude acid.
 11. A process according toclaim 5 in which the charge contains at least about 0.3 standard cubicfoot of carbon monoxide and at least about 0.2 standard cubic foot ofgaseous hydrogen per pound of said crude acid.
 12. A process accordingto claim 11 in which the molar ratio of carbon monoxide and hydrogen inthe charge is at least 10:1.
 13. A process according to claim 5 in whichsaid crude terephthalic acid is in the vapor state at an elevatedtemperature insufficient for the substantial decomposition ofterephthalic acid during said treatment, and the treated gaseous productmixture is thereafter cooled to condense at least a substantialproportion of the terephthalic acid vapor as a purified solid.
 14. Aprocess according to claim 13 in which said metal is suspended in finelydivided particulate form in the gaseous mixture during said treatmentand thereafter separated from said mixture prior to condensing theterephthalic acid.
 15. A process according to claim 13 in which saidcrude terephthalic acid vapor is mixed with a substantial amount of aninert gaseous substance.
 16. A process according to claim 15 in whichvaporized crude terephthalic acid is treated by contact at a temperaturebetween about 600*and 800* F. and a residence time between about 0.1 and5 seconds with an agent comprising between about 0.001 and 15 percent byweight of palladium supported in extended surface area form on finelydivided carbon particles in the presence of at least about 1 s.c.f. ofcarbon monoxide, at least about 0.5 s.c.f. of hydrogen and at leastabout 0.06 mol of steam per pound of said crude acid in a mixturewherein the carbon monoxide: hydrogen molar ratio is at least about20:1, and the resulting treated gaseous mixture is thereafter cooled inthe absence of said agent to condense at least a substantial proportionof terephthalic acid vapor as a purified solid.